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What is Canine Heartworm, and How Do I Know If My Dog Has Heartworm?

Canine heartworm is a potentially fatal parasite that infects dogs’ hearts, lungs, and blood vessels. The incidence of canine heartworm infection in the US has grown in recent years, expanding beyond the geographic areas (namely, the southeastern states) typically thought of as “danger zones.” Canine heartworm has now been diagnosed in all 50 states, thanks in part to more people traveling with their pets and adopting stray dogs from infected areas.

Once infected with heartworm, dogs can suffer lasting damage to their heart, lungs, and arteries; this can lead to severe lung damage, heart disease, and even death. Heartworms can infect dogs of all ages.

Here are some of the most common questions about detecting and treating canine heartworm:

How can my dog get heartworms? Canine heartworm can only be transmitted by mosquitos. When a mosquito bites an infected animal, it picks up bloodborne heartworm larvae, which it then transfers to other animals the next time it draws blood. These larvae then mature into adult heartworms, which can live for up to seven years in dogs. If a dog is infected with both male and female larvae, the heartworms can also mate inside their host; in this way, dogs can become infected with dozens, or even hundreds, of worms.

How do I know if my dog has heartworms? Dogs often show very few symptoms in the early stages of infection. The longer a dog is infected, the more symptoms will start to appear. Symptoms will be particularly severe in very active dogs, dogs with other existing health problems, and dogs who are infected with a large number of worms.Most dogs infected with heartworm will have a mild persistent cough and decreased appetite. They will experience weight loss and will get tired and winded after even moderate activity; many will lose interest in exercise altogether. As the disease progresses, some dogs may develop a swollen stomach due to a build-up of fluid in the abdomen. Untreated, heartworm disease can lead to heart failure or caval syndrome, which is a sudden blockage of blood flow inside the heart. Symptoms of caval syndrome include suddenly labored breathing, pale gums, and dark bloody or coffee-colored urine. If your dog presents with any of these symptoms, take him to the vet immediately.

Your vet will test your dog’s blood to detect the presence of heartworms. Some versions of these tests need to be sent out to labs for analysis, while others produce rapid results and can be read by your vet within 10-15 minutes. Your vet may also perform a chest x-ray or an echocardiogram (ultrasound of the heart) to confirm the diagnosis and determine how severe the infection is.

How is canine heartworm treated? A positive heartworm diagnosis can be very scary, but the good news is that heartworm in dogs can usually be treated very successfully, particularly if it is caught in the early stages. As soon as a positive diagnosis is confirmed, the first step will be to restrict your dog’s activity because any type of physical exertion will increase the damage done to the heart and lungs. This could mean anything from minimizing, or even eliminating, walks and playtime, to keeping your dog confined to a crate, to full hospitalization.
Your vet will next work to control your dog’s symptoms and make sure he is stable enough for treatment. This can include antibiotics to fight infection caused by the release of bacteria into your dog’s body as the heartworms die; a monthly heartworm preventive to kill smaller larvae; and corticosteroids to help reduce inflammation. Depending on the severity of the heartworm infection and your dog’s symptoms, this stabilization phase can take up to several months.Once your dog is stable, your vet will begin treatment to kill all adult and immature worms in his body. Treatment includes medication that is either injected into the dog’s back muscles or applied topically; the number of doses your dog needs will depend on the severity of the infection. These medications, while effective, can be toxic and can cause serious complications like blood clots in the lungs. Your vet will need to monitor your dog closely in the hospital after each injection or application. You dog’s blood will be retested after each treatment and six months after treatment ends, and may need additional rounds of medication if the infection persists.
In particularly severe cases, or in cases in which heartworm blockage has developed, dogs may require surgery to remove heartworms.

How can I protect my dog against heartworm? Prevention, prevention, prevention! Heartworm is almost 100% preventable in dogs, with very little effort. While there is currently no vaccine available to prevent heartworm, there are several preventive medications available, including a once-a-month chewable tablet, a once-a-month topical treatment, and a twice-a-year injection. As an added benefit, some of these medications help guard against other parasites, such as roundworms, hookworms, and fleas and ticks. Year-round use of preventive medication is recommended, regardless of where you live – heartworm has been detected northern states even during the winter!In addition to preventive treatment, it’s important to get your dog tested for heartworm annually as part of his routine wellness check-up. If your dog has not been on preventive medication previously, you will also need them to be tested prior to starting the medication and six and twelve months later. If you forget to give your dog his preventive medication for more than one month, he should also be tested before re-starting the medication.

How much will it cost to get my dog tested for heartworm? As with any veterinary cost, the price of testing your dog for heartworms will depend on your geographic area and your specific vet. Generally speaking, blood antigen tests to detect heartworm disease will cost anywhere from $40-100, plus the cost of the office visit. If your vet recommends an x-ray or echocardiogram, your bill could increase significantly – anywhere from $100-300 for each procedure.The cost of monthly heartworm prevention is very reasonable; a yearly supply will only cost you $30-80, depending on your dog’s weight. When it comes to heartworm, prevention is by far your most cost-effective option – once a dog is infected with heartworm, treating the disease can cost you upward of $1,000-2,000. As we’ve mentioned in other articles, pet insurance is a great option to help defray the cost of illness or accidents. Many insurance policies will even help cover your dog’s annual physicals, including annual heartworm tests. If you do find yourself facing a large out-of-pocket vet bill as your dog recovers from heartworm, CareCredit offers financing with no interest rate (for an agreed-upon period of time) for medical costs.

What types of canine heartworm test are there? The most common heartworm test used today is an antigen (aka serology) test, which detects microscopic pieces of heartworm skin in the bloodstream. This test has proven to be the most accurate and effective at detecting the presence of adult heartworms; one downside is that antigen tests can be less effective at detecting infections by younger worms (less than five months old). The different types of antigen tests include lateral flow tests, such as the SafePath® ZippTest® Canine Heartworm Test Kit, which can be done at your vet’s office, and the 96 well ELISA test, which requires blood samples to be sent to a reference laboratory.Prior to the development of these antigen tests, other tests looked for the presence of larvae in the bloodstream. These tests have largely gone out of fashion, though, because they were not very effective and often gave false negatives.

When it comes to keeping your dog safe from canine heartworm disease, prevention is key. Dogs should be started on monthly heartworm prevention no later than eight weeks of age and should remain on preventive treatment and get regular check-ups for their entire lives.